He said: “Charities do enjoy extraordinarily high levels of public trust. But there is a widespread failure among the public to understand how they operate. Some charities are huge organisations employing many thousands, others are tiny. Some spend a lot of their money on their core charitable activity, some very little.”

This is shown in research compiled by the Telegraph into the breakdown of spending by some of Britain’s biggest and most popular causes. In most cases, well over half of spending is on “charitable activity”, according to Charity Commission figures. But in two cases – the British Heart Foundation and Age UK – less than half of spending is defined this way.

Ratios change depending on the nature of a charity’s fundraising models. Those with large numbers of high street shops, for example, have to bear the costs of maintaining and staffing their premises.

Mr Saxton said: “For me, the Oxfam ratio looks about right, where three quarters of spending is on charitable activities.” But he said there were some doubts about how consistently “charitable activities” were defined across the sector. “There are many different kinds of beasts.”

Another issue is pay. Charities must disclose how many staff earn more than £60,000 but beyond that there is little disclosure. Mr Saxton said: “Is there a relationship between pay and performance? We don’t know.” His firm is currently researching this.